Field of the Invention
The subject application relates to a semiconductor device, a method for manufacturing a semiconductor device, a light-emitting device, and a method for manufacturing the light-emitting device.
Related Art
A transparent electrode is used in various fields, such as a Light-Emitting Diode (LED), a solar cell, a UV sterilizer for medical care, and fisheries, and application fields and the demand for the transparent electrode tend to gradually increase. In particular, the transparent electrode is chiefly used in the LED field. A current transparent electrode technology applied to an LED chiefly consists of an Indium Tin Oxide (ITO)-based technology which may be applied to a visible ray region (400 nm-800 nm) and some region (365 nm˜400 nm) of the entire UV region (10 nm-400 nm).
Recently, the demand for an UV LED that generates light of an UV region suddenly increases, but it is difficult to commercialize the UV LED because a transparent electrode having high conductivity and high transmittance in the UV region has not been developed so far.
For example, in the case of an UV LED having an ITO transparent electrode formed therein, which has been most used now, light of an UV region (10 nm˜320 nm) of a short wavelength generated from an active layer is mainly absorbed by ITO, and thus light externally extracted after passing through ITO is only about 1%.
FIG. 1 is a diagram showing transmittance in an LED structure according to a conventional technology.
This figure shows transmittance if a conventional ITO transparent electrode has been formed in a P-GaN semiconductor layer.
From FIG. 1, it may be seen that transmittance is 80% more in a region in which a wavelength is 350 nm or more, but transmittance rapidly decreases in an UV region of a short wavelength. In particular, transmittance decreases to 20% or less in a short wavelength region of 116 nm or less.
In another conventional technology for solving such a problem, a transparent electrode is not formed on a semiconductor layer, such as p-AlGaN, but a metal electrode pad is directly formed on the semiconductor layer. However, there is a problem in that an ohmic contact is not performed because a difference between the work functions of metal and the semiconductor layer is too great. Furthermore, there is a problem in that the amount of light generated from an active layer is significantly reduced because an electric current is concentrated on a metal electrode pad and is not supplied to the entire active layer.
In order to solve such problems, various researches are being carried out, but a transparent electrode having both high conductivity and high transmittance in an UV region has not been developed. The reason for this is that the conductivity and transmittance of a substance have a tradeoff relation. A substance having transmittance high enough to be used in the UV region has very low conductivity to be used as an electrode because it has a large bandgap, and thus cannot be used as an electrode because an ohmic contact with a semiconductor substance is not performed.
As an example of a technology suggested to solve such a problem, an application for a technology for forming a transparent electrode using a silver (Ag) thin film was filed as Korean Patent Application No. 10-2007-0097545. In such a conventional technology, however, if a transparent electrode is formed using silver (Ag), it is very difficult to thinly deposit silver (Ag) on a semiconductor layer so that an ohmic contact is performed. Although silver (Ag) is thinly deposited on the semiconductor layer, as shown in the graph of FIG. 4 of Korean Patent Application No. 10-2007-0097545, transmittance sharply decreases to 80% or less in a region in which a wavelength of light is 420 nm or less and transmittance decreases to 50% or less in a region in which a wavelength of light is 380 nm or less. There is no difference compared to transmittance of a conventional ITO electrode, which makes it difficult to expect the improvement of transmittance of an UV region.
Meanwhile, a light-emitting device, such as a Light-Emitting Diode (LED), had been used only in a limited field, such as light sources for display in home appliances in the early 1990s. Red, green, and blue LEDs capable of implementing high brightness and white light were developed based on the development of a new process technology and started to be used in the entire lift from the 2000s. The development of such an LED has an excellent environment-friendly property because the LED does not include an environment-harmful substance, such as mercury (Hg) used in existing lighting devices such as an incandescent lamp or a fluorescent lamp. It is expected that the LED will replace the existing light sources based on advantages, such as long lifespan and low power consumption characteristics.
An LED may be basically divided into a common type (or lateral type) light-emitting device and a vertical type (or thin GaN) light-emitting device in terms of its form. Furthermore, there is a flip-chip type light-emitting device, that is, a middle form of the common type and the vertical type.
The structure of the common type LED has a basic form including a single active layer that emits light and two cladding layers that surrounds the active layer on both sides thereof. The cladding layer coming into contact with an electrode may be subject to n-doping or p-doping. In general, one cladding layer portion coming into contact with a substrate is subject to n-doping and the other cladding layer portion is subject to p-doping. When a voltage is applied through an electrode according to the polarity of the doped cladding layer, the n-doped cladding layer supplies electrodes and the p-doped cladding layer supplies holes, thereby making an electric current flow. Accordingly, the electrons and holes are combined in the active layer, thus emitting light. In this case, the substrate is not separated, but remains intact. That is, in general, the common type LED has a structure in which an n type semiconductor, a quantum well, and a p type semiconductor are stacked on a substrate, etching is performed so that part of the n type semiconductor is exposed, a p type electrode is formed on the p type semiconductor, and an n type electrode is formed in the exposed n type semiconductor device.
The flip-chip type LED has a form in which the common type LED is turned over and fixed on a sub-mount through a stud bump, and is the same as the common type LED in terms of a basic structure for emission. In the flip-chip type LED having a relatively excellent heat-dissipation characteristic and high output characteristic compared to the common type LED, in general, light is emitted through the substrate.
The vertical type LED (VLED) also has the same basic structure for emission as the common type LED. In this case, an electrode is formed by separating a substrate itself from an n type semiconductor in order to expose the n type semiconductor without etching, instead of forming the electrode by exposing part of the n type semiconductor by etching. That is, a bonding/reflector and a receptor substrate are sequentially attached to a p type semiconductor device of an upper portion in the basic structure of the stacked common type LED, and the electrode is then formed. After the substrate of a lower portion is detached from the n type semiconductor device, the device is turned over and the electrode is then formed, thereby completing the basic structure of the vertical type LED. In other words, the vertical type LED has a form obtained by detaching the substrate in the common type LED structure and then turning over the device. Light emitted from the active layer is reflected vertically from a reflection plate at the bottom and then emitted toward an upper portion. The greatest advantage of the vertical type LED is a high heat-dissipation characteristic. Furthermore, the vertical type LED is advantageous in that emission efficiency is high compared to the common type LED because it has a vertical type structure of a thin GaN form from which the substrate has been removed.
As described above, the vertical type LED (VLED) attracts great interest due to advantages, such as an efficient heat dissipation plate and optical power improvements. However, in the present, it is essential to improve light extraction efficiency so as to fabricate a high-efficiency vertical type LED for the applications of a solid lighting device. The reason for this is that research of an electrode for improving light extraction efficiency is limited because the n electrode of the vertical type LED may have a problem, such as thermal damage, in a high temperature process. Furthermore, an LED manufacturing cost rises because additional processes, such as a Laser Lift Off (LLO) process for removing the substrate from the LED device and a reflection film deposition process, are required. Meanwhile, a polar problem, such as N face n-GaN, may be generated. Such a problem reduces the price competitiveness of an LED, and also deteriorates light extraction efficiency of the vertical type LED. Accordingly, there is a need for a technical solution for improving light efficiency of an LED by solving such problems.